Lesson 1: PIP, Gunicorn, Procfile, Requirements

1. Requirements Document (requirements.txt)

Purpose: Stores a list of all the libraries used in your project, including their version numbers.

Format:

Example → flask==3.1.0

(Uses two equal signs == to specify version.)

How it gets filled:

• When you install a library, it isn't automatically added.
• To update it, run:

pip freeze > requirements.txt

This captures all libraries from the virtual environment and writes them into the file.

Example: Adding MySQL

Install the library:

pip install mysqlclient

Update requirements file:

pip freeze > requirements.txt

Now you'll see mysqlclient listed inside requirements.txt.

Important Notes

• Using Tab Autocomplete helps avoid typos and ensures the file exists.
• Running pip freeze may capture many extra libraries in your virtual environment. That's normal—it just means everything needed for your project will be listed.

2. Pip (Python Package Installer)

Definition: Tool used to install libraries in Python.

Command format:

pip install <library_name>

Connection to requirements.txt:

Pip installs → pip freeze saves the list → requirements.txt keeps track of them.

3. Procfile

Location: Must exist in the root directory of your project.

Purpose: Tells Heroku which web server to run.

Format:

web: gunicorn app:app

• web: → process type (web server).
• gunicorn → server used by Heroku.
• app:app → points to your Flask app (app.py → Flask instance called app).

Important Notes

Procfile must:

• Start with a capital P.
• Have no file extension.
• Be spelled exactly as Procfile.

4. Gunicorn

• A lightweight Python web server.
• Required by Heroku to serve your Flask app.
• Must appear in requirements.txt (use pip install if missing):

pip install gunicorn

Key Takeaways

• requirements.txt: Tracks all libraries and versions (use pip freeze to update).
• pip: Installs libraries (e.g., Flask, MySQL, Gunicorn).
• Procfile: Instructs Heroku which server to run (gunicorn app:app).

Always double-check:

• gunicorn is installed and listed in requirements.txt.
• Procfile is formatted correctly and has no extension.